In their quest to understand how Complex IV would 'know' if cytochrome c was loaded up with an electron and 'ready' for the binding interaction, Imai et al. (2016) provided this 'surface' rendering of the Complex IV face that binds to cytochrome c:   It is color-coded with red to and blue to indicate negatively- and positively-charged surfaces respectively. (3D shape is important as well of course, and the article established that cytochrome c undergoes a 3D shape change as it gets reduced that could help it to better fit this binding site.)   This image has a large red patch on Complex IV indicating negative charge. Which parts of cytochrome c would be most likely to interact with this?   We didn't see all of the R-groups (especially the side chains) and potential additional backbone H-bond donors/acceptors when we were shown the 'front face' of cytochrome c, and we didn't look at its overall 'face' (contouring). However when cytochrome c is loaded with an electron (i.e. in its ______ state), one of the favorable interactions might be an electrostatic interaction with ______ that interacts with the Complex IV negative charge.   Two of the residues that were shown in an earlier figure (E4 and K13) are likely to be charged - review amino acid 1-letter codes and chemistry here Links to an external site. as needed so that you can fill in this blank. (Which one is positively charged? Which one is negatively charged? Therefore which one should be attracted to the negatively-charged Complex IV surface?)